1. Technical Field of the Invention
The present invention relates to signal processing and, more particularly, to the summation and integration of analog signals.
2. Description of the Related Art
The need to sum and integrate various analog signals is common in many applications. A common use is in frequency modulators, where several input signals need to be summed and integrated prior to introduction to a phase modulator. The integrator, however, has very high gain at DC, and most input signals will have a DC offset voltage associated with them. Hence, a technique for reducing DC offset levels is needed to prevent the circuit components in the integrator from saturating and to increase the dynamic range of the output signal.
A summer/integrator circuit is shown in FIG. 1 in accordance with the prior art. Two or more input signals 110 are first summed by the summer 115 as known in the art. The output of the summer 120 is tied to a DC blocking capacitor 130. Since a typical implementation of this circuit would be on an integrated circuit chip, and because the size required for the DC blocking capacitor would have be much larger than could be reasonably implemented on an integrated circuit chip, the DC blocking capacitor is a discrete capacitor, external to the integrated circuit chip. Connections from the DC blocking capacitor to the integrated circuit chip are provided by input and output pins 125 and 135 respectively. A resistor 140 is used to set the pole location for the high pass filter resulting from the resistor and the DC blocking capacitor. Finally, an integrator 150 as known in the art integrates the signal and provides an output signal 155.